Where an application involves a transmission with automatic gear shifting, especially in commercial vehicles, a backroll blocking agent, otherwise known as a “hill-holder”, is desired to provide security against reverse movement during a temporary stop. This desire arises especially from the fact that automatic transmissions are being increasingly used in those fields in which torque converter transmissions were previously employed. Because of the inherent characteristics of torque converters, however, these possess a tendency for forward creep, which hinders or at least minimizes backrolling.
Release devices for automatic transmissions have been made known, wherein an electronically controlled vehicle brake can be installed to function as a backroll blocker. Functional logic programming adds to the release of an energized vehicle brake; a signal of increasing torque of the frictional clutch binding the motor to the transmission. By a coincidence of a declining braking moment at the vehicle brakes and a simultaneous increase of traction-torque of the vehicle clutch, the function of backroll blocking is created.
EP 0 280 818 A1 discloses a braking system governing an apparatus, which recognizes the position of the vehicle and, following the establishment of a predetermined torque, which torque is transmitted from the motor to the vehicle wheels, a valve in the brake system opens, thus reducing the brake pressure, allowing the vehicle to again start forward.
In the manner of an example, DE 198 26 068 teaches of a transmission brake, which can be placed in a vehicle transmission, wherein it serves to assist in the synchronization of the located gear drive therein. Since the inertia of the transmission, this is carried out upon the synchronization of rotating parts, which rotates at varying speeds, can be braked through a shifting procedure through a transmission braking system with the result that the shifting procedure is accelerated. The transmission brake system is normally designed in lamellate fashion and is controlled by a pressure medium, usually oil, by means of a piston.
The purpose of the present invention is to provide the functionality of the backroll brake. At the same time, the purpose is to provide assurance that, even in the case of disturbances or failure of operation of the vehicle braking system, no critical condition can arise.